1. Technical Field
The present invention relates to acoustic sensors, and specifically to a MEMS (Micro Electro Mechanical Systems) type acoustic sensor manufactured using a MEMS technique.
2. Related Art
A capacitance type acoustic sensor is disclosed in Japanese Patent No. 4338395. In the acoustic sensor of Japanese Patent No. 4338395, a capacitor is configured by facing a thin film diaphragm and a fixed electrode film arranged in a back plate through a microscopic air gap. When the diaphragm vibrates by acoustic vibration, the gap distance between the diaphragm and the fixed electrode film changes by such vibration, and hence the acoustic vibration can be detected by detecting the change in electrostatic capacitance between the diaphragm and the fixed electrode film.
In such a capacitance type acoustic sensor, the diaphragm sometimes fixes to the fixed electrode film (hereinafter a state or a phenomenon in which the diaphragm is partially or substantially entirely fixed to the fixed electrode film so that the gap is eliminated is called a stick) in the manufacturing process or during use. When the diaphragm sticks to the fixed electrode film, the acoustic vibration cannot be detected by the acoustic sensor because the vibration of the diaphragm is inhibited.
The cause of sticking in the acoustic sensor is as follows (details in Japanese Unexamined Patent Publication No. 2008-301430). In the manufacturing process of the acoustic sensor such as a cleaning process after etching a sacrifice layer, moisture infiltrates to the air gap between the diaphragm and the fixed electrode film. Moreover, moisture may infiltrate to the air gap between the diaphragm and the fixed electrode film due to humidity and water wetting even during the use of the acoustic sensor. Meanwhile, the gap distance of the acoustic sensor is only a few micrometers and furthermore, the diaphragm only has a film thickness of about 1 μm and has a weak spring property. Thus, if moisture infiltrates to the air gap, the diaphragm gets adsorbed to the fixed electrode film (this is a first stage of sticking) by capillary force or surface tension, and such a sticking state of the diaphragm is held by the inter-molecular force, inter-surface force, electrostatic force, and the like acting between the diaphragm and the fixed electrode film even after the moisture has evaporated (this is a second stage of sticking).
The first stage of sticking may also occur, such as the diaphragm may attach to the fixed electrode film when a large sound pressure or drop impact is applied on the diaphragm thereby greatly displacing the diaphragm, or the diaphragm may attach to the fixed electrode film when charged with static electricity.
An acoustic sensor in which a great number of stoppers (projections) are arranged on a surface facing the diaphragm of the fixed electrode film to prevent the sticking of the diaphragm is known. Such an acoustic sensor is disclosed in Japanese Unexamined Patent Publication No. 2006-157863, and the like.
FIG. 1 and FIG. 2 is a plan view and a cross-sectional view each showing an acoustic sensor including a stopper. In FIG. 1 and FIG. 2, a mode similar to the acoustic sensor of a first embodiment of a conventional example is shown to facilitate the comparison with a first embodiment of the present invention. In an acoustic sensor 11, a back chamber 15 passes through from an upper surface to a lower surface of a silicon substrate 12, and a diaphragm 13 of thin film form made of polysilicon is arranged on the upper surface of the silicon substrate 12 to cover an upper surface of the back chamber 15. Furthermore, a canopy shaped back plate 14 is fixed to the upper surface of the silicon substrate 12 so as to cover the diaphragm 13. The back plate 14 has a fixed electrode film 20 made of polysilicon arranged on a lower surface of a plate portion 19 made of SiN. A microscopic air gap is formed between the diaphragm 13 and the fixed electrode film 20, where the diaphragm 13 and the fixed electrode film 20 configure a capacitor. A great number of acoustic holes 18 for passing the acoustic vibration are opened in the entire back plate 14. A plurality of stoppers 22 each having projection shape of the same length and the same diameter are arranged at substantially equal intervals over the entire lower surface of a region facing the diaphragm 13 of the back plate 14.
According to such an acoustic sensor 11, a distal end face of each stopper 22 comes into contact with the diaphragm 13 even when the diaphragm 13 is greatly displaced thereby preventing the diaphragm 13 from overly approaching the back plate 14 and inhibiting the sticking of the diaphragm 13.